Transformer and transformer system



June 16, 1931. F. E. FIELD TRANSFORMER AND TRANSFORMER SYSTEM Filed June 19, 1 25 H?! Ai----- c Patented June 16, 1931 UNITED STATES FRANK E. FIELD, OF SOMEEVILLE, NEW JERSEY, ASSIGNOR TO WESTERN ELEC'IRIQv PATENT OFFICE COMPANY, INCORPORATED, OF NEW YORK, N. Y., A CORPORATION OF NEW YORK TRANSFORMER AND TRANSFORMER SYSTEM Application filed June 19,

This invention relates to transformers and i more particularly to differential, or bridge,

transformers for use in connection with simplexed transmission systems.

An object of the invention is to improve the transmission efliciency of differential transformers.

Another object is to reduce the effective leakage inductance of the differential windings of such transformers while at the same time maintaining a low electrostatic capacity between the windings.

The invention is applicable to simplexed transmission systems, such as phantomed telephone circuits, and to the so-called duplex modulating systems of the type described in U. S. atent No. 1,343,308 to J. R. Carson issued June 15, 1920.

In thisconnection further objects of the invention are to facilitate the superposition of currents in simplexed systems, and to improve the transmission eificiency of systems of this type. I

A feature of the invention lies in the couplingof the leakage magnetic fields of the transformer windings with a short-circuited winding, the reactions of-which due to induced circulating currents cause the leakage inductance to be substantially neutralized.

A further object is therefore to utilize the effectof induced circulating currents to neutralize the leakage inductance of transformer windings.

The nature of the invention will be more fully understood from the following de tailed description taken in conjunction with the accompanying drawings, of which:

Fig. 1 shows in simplified schematic form a simplexed transmission system embodying the invention;

Fig. 2 shows the application of the tion to a modulating system; 1

Fig. 3 shows in detail the winding arinvenrangement of the transformers employed in the circuits of Figs. 1 and 2; and

Fig. 4 is a simplified diagram for purposes of analysis.

Referring to Figs. 1, 5 and 6 are a pair of line wires adapted for simplex operation by means of differential transformers T and T 1925. Serial No. 38,124.

through which two separate sources of currents S and S are connected over the line wires to receiving impedances 7 and 8 respectivel 'lransfhrmer T is provided with balanced secondary windings 1 and 2 which are connected in series, inductively aiding, and with two equal primary windings 3 and 4 which are connected in parallel, inductively aiding.

Transformer T is of similar type and is pro-- vided with corresponding windings designated 1, 2, 3 and 4, respectively.

Source S is connected to the terminals of the primary windings of transformer T and receiving impedance 7 is similarly connected to transformer T thereby establishing a simple series transmission circuit over the wires 5 and 6.

Source S is connected between the midpoint of windings 1 and 2 and ground, and

receiving impedance 8 is connected in like manner between windings 1, 2 and ground. Currents from S are thereby superimposed upon the lines in parallel, returning from impedance 8 to the source by way of the ground. If the lines are of equal impedance, as is generally the case, the superimposed current divides equally between them, the two parts flowing through the transformer windings in opposite directions, and producing a differential magnetic. effect which is practically zero except for the effect of magnetic leakage.

For the eflicient transmission of the superimposed current, the two series-connected windings of each transformer should be very closely coupled together so that the leakage impedance of the parallel connection is reduced to a minimum.

A common method of securing this result' generally result in the introduction of larger leakage inductances, so that the transmission of the superimposed. current is impaired.

According to the invention the leakage inductance of the dillerentia'l windings is reduced without causing the introduction of a large shunt capacity by means of a special arrangement or the transformer windings. This winding arrangement is illustrated in Fig. 3 which shows in detail the physical construction of one of the transformers, such as T The transforn'ier comprises a. shell type magnetic circuit 9 built up of laminated magnetic material such as iron or silicon steel, around the central core of which is placed a spool 10 of insulating material containing the wiiidii'igs. The windings are applied in six sections which are arranged in two groups of 3 concentric coils each, the individual sections being designated in the figure by numbers corresponding to the designations of the windings in Figure 1. The primary windings 3 and l occupy the two middle sections of each concentric group, secondary winding 1 is divided equally between the outer and inner sections of the one group, and secondary winding 2 is divided between the outer and inner sections of the other group. All sections are wound in the same direction. The interconnection of the sections and the method of connecting the transformer in the transmission circuit are indicated by the numbers 12, 13, 1 1, 15 and 16 which designate 'correspon ding terminals in both Figs. 1 and 3.

It will be seen that winding 1 is unequally coupled to windings 3 and l, beingmore closely coupled to winding 4 than to 3. Similarly winding 2 is more closely coupled to winding 3 than to 1-. lVith respect to the main flux threading the magnetic circuit the windin s are of course equally coupled and the (lifi'erences in the degrees of coupling be tween the individual windings are due to the leakage fluxes which surround them without traversing the n'iagnetic circuit.

Further, although windin, i;s 1 and L are unsymn'ietricallv coupled to windings El and 4. individually, the arrangei'ncnt is of such s unmetry that they are equally coupled to the parallel combination of windings 3 and llhen equal currents from source S; traverse windings 1 and 2 in opposite directions the current in winding 1, by virtue ot the unbalanced COUpllDg, induces an unbalanced E. M. F. in windings 2-3 and t tending to produce a current in winding 4t in the direction, for example, from terminal 13 to terminal 12. Similarly, the current in winding 2 produces an unbalanced E. M. F. tending to send a current through winding 8 in the opposite direction. The directions of these induced currents and of the inducing currents in windings 1 and 2 are indicated in the figure by the arrows associated with the respective windings.

The induced currents circulate in the same direction in a closed path constituted by win-dings 3 and 4; connected in series, and since these windings are magneticall aiding when connected in parallel, their series connection is practically non-inductive. To the unbalanced induced E. M. F. the windings therefore act in the same manner as a shortcircuited secondary winding in a transformer and hence tend to neutralize the mutual inductance by which the induced E. M. Es are created, that is, a substantial portion of the leakage inductance of windings 1 and 2 in their parallel connection.

The neutralizing action may be quantitatively analyzed by a consideration oi the simplified circuit shown in Fig. i. This ligure corresponds to part of Fig. 1, comprisin transformer T and source S line wires 5 and 6 being disconnected and terminals 14: and 15 being connected together and to the ground tap. Under these conditions current from the source S encounters only the open circuit impedance of the transformer due to the effective leakage impedanccs.

The currents in win-dings 1 and 2 are designated I and L respectively and the circulating current in windings 3 and 4; by 1 The total current from the source S, is designated by L, which is equal to the sum of l} and I The following equations, based on Kircho'JTs laws serve to ('leterininate the value of the four currents 3 34l( 1 12)( 14 13) l The mathematical. comlitions corispending to the specified symmetry being expressed by the equation Equation 2 shows that the circuit is equivaloss, increases more or less tual impedance Z,,,, the values of these impedances being given by the equations:

The impedances constituting the equivalent coupled circuits are evidently the leakage impedances of the transformer windings, and the mutual impedance is that due to the coupling of the leakage fields.

Equation 2 may be converted to the form 2E 1712) 1 (5) in which is is the coefficient of coupling, defined by i age inductance is efiectively destroyed.

The invention is of particular utility when the system upon which the currents are superimposed is of high impedance, for in that case highly inductive secondary windings using a large amount of wire must be employed, and if recourse is had to the bifilar method of winding the two secondaries, the capacity between them becomes excessively large. In

fact, the loss in the series circuit due to the shunt capacity increases very rapidly as the impedance of the system is increased, due to the fact that shunt admittance, causing the proportionately to the impedance.

Fig. 2 shows the invention applied to a differential modulating system of the type described in U. S. Patent 1,343,308 to J. R. Carson issued June 15, 1920. Systems of this type are in general of. much higher impedance than the ordinary simplexed transmission systems such as telephone'systems.

The system is of the same general type as that shown in Fig. 1 with the addition of an asymmetric coupling device, in the form of two three element space discharge tubes 17 and 18', which divides the line into two parts.

These parts are denoted by 5, 6, and 5, 6 to correspond with Fig. 1.

The space discharge tubes are of the customary type in which an electron stream between a cathode and an anode is controlled by the potential of a third, or control, electrode. Cathodes l9 and 20 are connected together and to ground, control electrodes 21 and 22 are connected respectively to lines 5 and 6, and anodes 23 and 24 are connected respectively to line wires 5 and 6. Space current is provided by a common battery 25 which is connected in a circuit between the mid-point of the secondary windings oftransformer T and ground.

The system is arranged for modulation. Waves of one frequency from source S are impressed upon lines 5 and 6 in series, thereby energizing the control electrodes in oppo site phases. These waves arerepeated in the space paths of the tubes and the repeated waves are impressed upon lines 5 and 6 in series and thence through transformer T to load 7. 7"

Vaves of a different frequency from source S are impressed upon the control electrodes in parallel, thereby energizing the control electrodes in the same phase; The repeated waves are also in the same phase, and are impressed upon lines 5 and 6 in parallel and thence in opposition through the secondary windings of transformer T to ground.

When the two sets of waves traverse the space paths of the discharge tubes simultaneously, modulation E. M. F.'s are generated therein, the principal modulation prod ucts being those of frequencies corresponding to the sum and to the difference of the impressed wave frequencies.

These products, known as second order modulation products, are of opposite phase in the two tubes and hence are impressed upon the lines 5' and 6 in series whereby they are transmitted by transformer T to load 7.

The system may be used or for demodulating, the two identical in principle. Thus, S may be a" source of speech frequencies and S a carrier wave source giving a wave of kilocycle frequency, in this case both the speech frequency waves and the modulation products beingtransmitted to impedance 7 while the carrier wave is suppressed in the output by the differential action of the output transformer. The modulation products comprise two bands of side frequencies, one comprising waves of the difference frequencies be tween the carrierand speech frequencies, and the other comprising the corresponding summation frequencies.

Again, the source S may be a line from which is received one of these bands of side frequencies and S a local source of carrier waves of appropriate frequency to cooperate in the reproduction of speech or signal currents from the side bands. In thisv case themodulation products include detected speech for modulating processes bemg currents which along with the repeated side band waves are transmitted to the impedance 7.

In each case, if it is desired, the repeated waves from source S may be prevented from reaching the load impedance by inserting in front of the load impedance a wave filter or other selective network such as shown in U. S. Patent 1,227,113 to G- A. Campbell, May 22, 1917.

In order that the modulation E. M. F.s generated in the space paths may have their maximum value it is necessary that the repeated currents due to the impressed waves should be as large as possible, and to secure this it is desirable that the repeated waves should encounter substantially zero impedance in the output circuit external to the tubes.

The principles leading to this conclusion are developed in U. 55. Patent 1,448,702 to J. E. Carson issued March 13, 1923 and in U. S. Patent 1,624,498 to F. Mohr, issued April 12, 1927. This condition is secured with respect to the waves repeated from source S as a result of the winding arrangement of transformer T in accordance with the invention.

\Vhen the system is adapted to modulate a carrier wave from source S by a speech wave from source S the windings of transformer T being proportioned for the eflicient transmission of the relatively high frequency side bands, would generally have low impedances at speech frequencies and would have little effect in limiting the repeated speech currents.

When the system is adapted to reproduce speech signals by demodulation it might appear that it would be desirable to have a large capacity between the transformer secondary windings in order that the high frequency "aves repeated from source S, may have a low impedance path in the output circuit, but it is also necessary that such capacity be very carefully proportioned to avoid excessive loss to the transmission of the detected speech currents, and the proper capacity value can be provided with much greater certainty by means of a separate condenser. In practical systems, it would commonly be the case that such capacity would form part of a filter or other selective networl; and in consequence its value would be determined by a number of design requirements. For this rem-ion, it is desirable to eliminate the capacity of the windings, completely if possible.

The nature and the mode of employing the invention in specific cases having been dis closed, the scope of the invention is defined hy the following claims.

What is claimed is:

1. In combination, in a simplexed transmission system, a pair of line wires, a third line constituting a return path for waves superimposed upon said line wlres in parallel, a

transformer, a pair of windings on said transformer connected in series inductively aiding between said pair of line wires and having their common point connected to said third line, and a second pair of windings on said transformer connected in parallel, inductively aiding, said windings being applied in a pin 'ality of sections on a common magnetic core and said sections being symmetrically interleaved whereby the impedance of said series connected windings to superimposed waves is neutralized at least in part by virtue of their resultant coupling with the closed circuit constituted by said parallel con nected windings.

2. In combination, in a simplexed trans mission system, a pair of line wires, a third line constituting a return path for waves superimposed upon. said line wires in parallel, a traus'lknimzr, a pair of windings on said transformer connected in series inductively aiding between said pair of line wires and having their common point connected to said and a second pair of windings on d transformc' connected in parallel, inctireiy aiding, said windings being so inte. ieaved upon a common core that the leaka ge inductances of said series connected windin with respect to simplex currents are subetanti ally neutralized by the action of circulating currents in said parallel windings.

3. In a simplexed transmission system, a pair of line wires, a t'ansformer having a pair of balanced windings connected in series, inductively aiding between said line wires. :1 tap at the common connect-i on of said windings, a third path connected to said tap and constitutin a return circuit for waves superimposed upon said line wires in pa allel, and means for reducing the impedance of said transformer to superimposed cm'rents, said means comprising a short-circuited path inductively coupled to said balanced wimlings.

i. In a simplexed transmission system. a transfm-mer con'iprising a pair of windings connected in series, magnetically aiding, the terininals of said series pair being connectzul to a pair of line wires and their common point to the third line of the siinplcxod system, a second pair of windings on said transformer, connected in parallel, magnetically aiding, each of said latter windings being in dividually interleaved with one of said first windings and more completely linked y the leakage flux thereof than by the leakage flux of the other of said first windings.

5. In combination, in a simplexed transmission system, a pair of line wires, a third line constituting a return path for waves superimposed upon said line wires in parallel, a transformer con'iprising a pair of windings connected in series inductively aiding between said pair of line wires and having their common point connected to said third line, and con'iprising also a second pair of windings connected in parallel, inductively aiding, said first windings being each closely magnetically associated with an individual one of said latter windings and less closely magnetically associated with the other of said latter windlngs.

6. In a simplexed transmission system a pair of line wires, an inductance element having a pair of balanced windings connected in series, inductively aiding between said line wires, a tap at the common connection of said windings, a third path connected to said tap and constituting a return circuit for waves superimposed upon said line wires in parallel, and means for reducing the impedance of said inductance element to simplexed currents comprising a short-circuited path linked with the leakage flux of said balanced windlIlgS.

7 In a simplexed transmission s stem comprising a pair of line Wires and a t ird return wire, a differential transformer having a pair of windings connected across said line Wires in series, inductively aiding, with their common point connected to said return wire,

and a second pair of windings connected inparallel, inductively aiding, each of said latter windings being interleaved with an associated first winding to link the leakage flux thereof and connected in a closed path for dissipating the energy represented by said leakage flux.

In witness whereof, I hereunto subscribe my name this 9th day of June A. D., 1925.

V FRANK E. FIELD. 

